Visible-wavelength interferometers are widely employed for high precision displacement measurement and surface profilometry. However, a well-known problem with these instruments relates to the interface phase ambiguity. In that the speckle pattern generated by visible laser light on a rough surface has an essentially random phase content with a standard deviation larger than 2.pi., conventional interferometry does not normally yield any useful information about the profile of a rough surface. As a result, mechanical gauges are often used in place of interferometers in precision machining, inspection and optics manufacture. However, it is not always desirable or physically possible to mechanically contact a surface in order to perform a profile measurement.
One known method to extend the range of metrology applications for interferometry is to measure the interferometric phase at two distinct wavelengths. The difference between the two phase measurements corresponds to a synthetic wavelength (.LAMBDA.) given by EQU .LAMBDA.=.lambda..sub.1 .lambda..sub.2 /(.lambda..sub.2 -.lambda..sub.1)
where .lambda..sub.1 and .lambda..sub.2 are the two distinct optical wavelengths. In that the synthetic wavelength may be very large, compared to visible-light wavelengths, it is possible to accommodate profile discontinuities and surface roughness that would be beyond the capability of a conventional interferometer.
As described in U.S. Pat. No. 4,832,489, issued May 23, 1989, to J. C. Wyant et al., a two-wavelength phase-shifting interferometer employs two laser sources for reconstructing steep surface profiles, such as aspheric surfaces. A 256.times.256 detector array is used and the technique computes an equivalent phase independently for each detector.
The following articles discuss various aspects of employing a synthetic wavelength for surface profilometry.
In an article entitled "Contouring Aspheric Surfaces Using Two-Wavelength Phase-Shifting Interferometry" by K. Creath, Y. Cheng, and J. Wyant, Optica Acta, 1985, Vol. 32, No. 12, 1455-1464 there is described two-wavelength holography using an argon-ion laser and a He-Ne laser. An uncoated aspheric surface was placed in one arm of an interferometer and synthetic wavelengths of 2.13 micrometers and 2.75 micrometers were employed. Interferograms were recorded using a 100.times.100 diode array. Primary interferograms were manipulated by a computer to produce a secondary interferogram from double-exposure measurements.
In an article entitled "Absolute Optical Ranging with 200-nm Resolution" by C. Williams and H. Wickramasinghe, Optics Letters, Vol. 14, No. 11, Jun. 1, 1989 there is described optical ranging by wavelength-multiplexed interferometry and surface profiling said to be carried out on an integrated circuit structure. A pair of GaAlAs single-mode diode lasers are used as optical sources.
In an article entitled "Two-wavelength scanning spot interferometer using single-frequency diode lasers" by A. J. de Boef, Appl. Opt., Vol. 27, No. 2, Jan. 15, 1988 (306-311) there is described the use of two single frequency laser diodes to measure the profile of a rough surface. The two wavelengths are not time-multiplexed but are instead continuously present.
In an article entitled "Two-Wavelength Speckle Interferometry on Rough Surfaces Using a Mode Hopping Diode Laser" by A. Fercher, U. Vry and W. Werner, Optics and Lasers in Engineering 11, (1989) pages 271-279 there is described a time-multiplexed two-wavelength source consisting of a single mode diode that is switched between two adjacent oscillation modes. The switching is accomplished by pump-current modulation with the diode thermally tuned to a region near a so-called "mode hop", that is, near a region where the diode output readily switches from one wavelength output to another. This technique is said to have enabled the profiling of a ground lens surface having an estimated surface roughness of four micrometers.
These authors report that if the standard deviation of the surface profile is larger than the single wavelengths a fully developed speckle field is obtained. Amplitudes and phases of the speckle field are determined by the microscopic structure of the reflecting surface, but no deterministic relationship exists between the phases of the single-wavelength interferograms and the surface or the distance to be measured. If, however, two wavelengths are used the phase difference between the corresponding speckle fields contains not only information about the microscopic surface roughness but also about the macroscopic surface profile or distances. This information is accurate provided the standard deviation of the microscopic surface profile is smaller than the effective wavelength.
However, single-mode diodes, such as those described in certain of the above referenced articles, must typically be burned-in and carefully characterized in a monitored environment, especially if controlled switching between oscillation modes is required. Although multiple wavelength interferometry is a known technique with many applications relevant to modern metrology problems, its use is not widespread due, in part, to a difficulty in obtaining practical, reliable and inexpensive multiple-wavelength sources and detectors.
It is thus an object of the invention to provide apparatus to accomplish profilometry by exploiting a multimode behavior of a relatively inexpensive multimode laser diode.
It is a further object of the invention to provide apparatus to accomplish rough surface profilometry that avoids the use of single-mode laser diodes and/or a requirement that two wavelengths be time-multiplexed.